1. Field of the Invention
The present invention generally relates to a bandgap circuit, and more particularly to an auxiliary control circuit for the bandgap circuit.
2. Description of Related Art
A voltage reference is an electronic circuit that generates a fixed voltage regardless of the loading on the circuit. A bandgap circuit is one of the voltage reference circuits for generating a fixed reference voltage that has a value equal to the electron bandgap level of silicon (approximate 1.2 volts) and changes very little with temperature. The bandgap circuits are widely used in electronic systems. FIG. 1, for example, shows a bandgap circuit 101 used in the source driver 10 for a liquid crystal display (LCD) panel 12. A mirror circuit 103 mirrors the current in the bandgap circuit 101. The bandgap circuit 101 and the mirror circuit 103 constitute portion of the power circuit 100 of the source driver 10. The output of the mirror circuit 103 is fed to buffers of driving channels 102. It is well known that a self-biased circuit such as the bandgap circuit 101 shown here, during a start-up phase, has an undesirable zero-bias state in which zero current flows in the circuit. In order to obviate this problem, a start-up circuit 105 is thus needed.
An ideal start-up circuit should not affect the bandgap circuit 101. In other words, the start-up circuit should be inactivated, and the current through the start-up circuit should become zero or very small during normal operation (or after start-up phase). It is, however, unfortunately found that most of the conventional start-up circuits 105 did affect the bandgap circuit 101. Specifically, after the positive power supply VDDA has reached a specified level and during the normal operation, the component or components in the start-up circuit 105 are not completely close or shut down as required. As these components are leaky, they cause unwanted increase in current in the bandgap circuit 101. Worst of all, the output currents of the mirror circuit 103 would increase abruptly under a positive power supply VDDA greater than a specified value. Such an increased current disadvantageously incurs higher consumed power, and at the worst, the functions of circuit stage receiving the output currents would consequently fail.
For the foregoing reason, a need has arisen to propose a scheme to control the start-up circuit 105 such that the start-up circuit 105 does not affect the bandgap circuit 101 during the normal operation.